This invention relates to an ultrasound apparatus and method for amplifying transmit signals. In particular, the invention relates to amplifying pulse wave and continuous wave signals for application to a transducer element.
In ultrasound, continuous waves (CW) are associated with spectral Doppler modes of operation, and pulse waves (PW) are associated with pulsewave spectral Doppler, B-mode and Color Doppler modes of operation. Continuous waves are waves that repeat cyclically for a plurality of cycles, such as 50-100 cycles. Continuous waves provide little or no range resolution. Pulse waves provide range resolution and are associated with only a few cycles of a waveform, such as 1-2 cycles. Longer duration PW operation or shorter duration CW operation are also possible.
During PW or CW operation, a plurality of transmit waveforms are applied to a respective plurality of transducer elements. Each transducer element is associated with a channel or a processing path for generating the transmit waveform.
Many ultrasound systems are capable of both CW and PW operation. These systems include analog or digital structures for generating appropriate waveforms (PW or CW). For example and as disclosed in U.S. Pat. No. 5,675,554 (the '554 patent), the disclosure of which is herein incorporated by reference, digital samples representing a transmit waveform are generated at a particular sample rate. The samples are converted to analog differential currents representing the transmit waveform.
Before application of the transmit waveform to a transducer element, the waveform is amplified. In particular, the transmit waveform is current and voltage amplified by current and voltage amplifiers. Regardless of CW or PW operation, the transmit waveforms are applied through the same current and voltage amplifiers. Thus, the voltage amplifier is powered from a programmable or variable power supply. The supply voltage is set as a function of the peak output voltage.
For CW operation, the supply voltage is significantly less than for PW operation. The acoustic waveform generated by the transducer element is limited by government regulation of the power generated at various points along the path of propagation. Due to the lesser duration during PW operation, the pulse waveforms may have higher peak powers (higher current and voltage). Due to the longer duration during CW operation, the continuous waveforms have lower power (lower current and voltage).
Depending on the mode of operation (CW or PW), the programmable power supply is adjusted. For example, the programmable power supply provides a voltage within a 30 to 210 volt range in selectable steps, such as 15 volt increments with finer steps in the 30-40 and 100-210 volt ranges. However, implementing a programmable power supply requires expensive hardware.